Vessel for transporting low temperature liquids



y 1, 1962 J. 5. WIEDEMANN ETAL. 3,031,856

VESSEL FOR TRANSPORTING LOW TEMPERATURE LIQUIDS 3 Sheets-Sheet 1 FiledMarch 24, 1961 John S. Wiedemcmn Inventors By We??? Ari'or'ney Edward B.Schumucher May 1, 1962 J. s. WIEDEMANN ETAL 3,031,856

VESSEL FOR TRANSPORTING LOW TEMPERATURE LIQUIDS 3 Sheets-Sheet 2 FiledMarch 24, 1961 i any mh mm mm xa ea E 0% mm 8 r o n e V .m. r e m m 0 mm h M 0 .m w B m G n .n W 0 d u E By i Putnt Attorney May 1, 1962 J. 5.WIEDEMANN ETAL 3,

VESSEL FOR TRANSPORTING LOW TEMPERATURE LIQUIDS Filed March 24, 1961 3Sheets-Sheet 3 John S. Wiedemunn Edward B. Schurr mcher Patent AttorneyInventors 3,031,856 Patented May 1, 1962 ice 3,031,?)56 VESSEL FURTRANSPBRTKNG LGW TEMPERA- TURE LIQUES John S. Wiedemann, Staten island,N.Y., and Edward B.

fichuntacher, lvlatawan, NJ assignors to Esso Research and EngineeringCompany, a corporation of Delaware Filed Mar. 24, 1961, Ser. No. 99,01111 (Ilaims. (Cl. 62-45) This invention relates to means for transportinglow temperature liquids in bulk. It relates particularly to ship meansfor bulk transportation of low temperature liquids. it relates moreparticularly to a vessel or tank ship construction suitable for bulktransportation of low tempera ture liquids at substantially atmosphericpressure, and it relates still more particularly to a vessel or tankship construction suitable for bulk transportation at substantiallyatmospheric pressure or" liquid materials having normal (atmospheric)boiling points down to about F.

This application is a continuation-in-part of an application Serial No.824,427, filed in the United States Patent Otiice on July 1, 1959, nowabandoned.

Considerable interest has been shown in recent years in the storage andtransportation of low-boiling hydrocarbon materials such as propane(-43.7 F. normal boiling point) in the liquid state and at substantiallyatmospheric pressure. Under these conditions the cold hydrocarbons areplaced in thermally insulated contain ers and allowed to vaporize orboil oil as heat leaks in through the container structure. The vaporsthus produced may be either vented directly to the atmosphere, consumedas a gaseous fuel, or recondensed by suitable refrigeration equipmentand returned as liquid directly to the insulated container. Obviouslythe efficiency and economy of such storage and transportation oflow-boiling liquids are dependent to a large degree upon theeffectiveness of the thermal insulation applied to the liquid container.

Another problem besides that of vaporization loss of stored materialswhich arises in the course of storing and transporting low-boilingliquids is that of embrittlement of metallic structural components ofthe liquid container. At temperatures of the order of that of liquidpropane at atmospheric pressure (-43.'7 R), ordinary ferrous materialssuch as low carbon steel may suffer at least some loss of energyabsorbing capacity at high rates of strain, that is, loss of impactresistance. Storage tanks in general, and storage tanks aboard ship inparticular, may be expected to be exposed to some shock loads duringtheir working lives, even though such loads be applied accidentally.Accordingly, when low-boiling materials such as propane are liquefiedfor storage and/or transportation in bulk thought must be given to thematter of loss of impact resistance of a steel-walled container and theattendant increased susceptibility of this structure to brittlefracture.

It will be readily understood that the physical failure of a wall of atank containing cold liquid hydrocarbons in bulk, liquid propane forexample, could be extremely dangerous to both life and property. Tominimize the possibility of such failure, proposals have been made forstoring liquid propane and other cold materials in steel tanks orcontainer shells provided with internal insulation of substantialthickness. By placing the insulation on the inside of the steelcontainer shell rather than on the outside, the shell material isallowed to remain at substantially atmospheric temperature for itsentire thickness even though the container be fully charged with coldliquid. In this way, low carbon, relatively inexpensive steels may beused in the tank structure instead of resort having to be had to costlyalloy steels or other materials retaining significant impact resistanceproperties at low temperatures. The insulating materials which have beenproposed for such use include balsa wood and cork in the form of rathersizable blocks or slabs.

Whether the insulation comprise blocks or slabs of the traditionalmaterials or be otherwise constructed, however, direct exposure of theinsulating material to the cold liquid being stored or transported hasusually not been desired. Accordingly, a liner or inner tank shell isplaced within the insulating blocks. One possibility known to the priorart for creation of this inner tank shell. is to have it made ofaluminum or another metal such as stainless steel which retains asignificant impact resistance property at low temperatures. Forshipboard containers intended to hold liquids at very low temperatures,for example, containers to hold liquid methane at about -259 R, an innertank shell of proper metallic material is the only practicalpossibility. However, when liquids at about the temperature of liquidpropane, that is, at about -50 F, are to be carried the inner tank shellmay be non-metallic and susceptible of easier handling and installationthan a metal. Likewise, in this temperature range, the insulation blocksmay be of material other and less expensive than and at least as easilyinstalled as the balsa or cork of the prior art.

According to the present invention, a vessel for transporting lowtemperature liquids is provided in which the inner one of two cargoliquid tank shells both independent of thehull structure of the vesselcomprises a polyester sheet material.

Further, according to this invention, a vessel of the kind described isprovided in which the insulation between the two inner and outer tankshells is a prefoamed plastic material.

Still further, according to this invention, a vessel of the kinddescribed is provided in which the inner and outer tank shells and theinsulation therebetween are bonded in leak-proof fashion by a Thiokol-epoxy sealing material.

Even still further, according to this invention, means are provided forconveniently loading cold cargo liquids into and discharging suchliquids from the insulated storage tanks of the above-described vessel;for either recondensing and preserving or else distantly venting thevapors boiled oil from these storage tanks, and for rendering inert theatmosphere around the outer shell of any tank structure to preventcreation of combustible gaseous mixtures in this atmosphere upon theleakage thereinto of vaporized cargo materials.

The nature and substance of this invention may be more clearly perceivedand fully understood by referring to the following description andclaims taken in connection with the accompanying drawings in which:

FIG. 1 represents a side elevation view, partially broken away, of atank vessel designed to carry liquefied, normally gaseous materials,such as liquefied petroleum gas according to this invention,particularly illustrating in schematic form the piping systems forhandling the cold liquid cargo material and the vapors generatedtherefrom;

FIG. 2 represents a deck plan view of the tank vessel of FIG. 1,likewise partially broken away;

PEG. 3 represents a transverse sectional elevation view of the tankvessel embodying this invention taken along line 3-3 in FIGS. 1 and 2 inthe direction of the arrows, particularly illustrating features ofsupport and internal construction of an insulated container for coldliquid cargo material, and

FIG. 4 represents a schematic diagram of the refrigeration apparatusprovided to recondense vapors generated from the cold liquid cargomaterial carried in insulated containers in the tank vessel of FIGS. 1and 2.

Referring now to the drawings in detail, especially to FIGS. 1 and 2thereof, a marine vessel externally configured more or less similarly toa conventional tank ship is designated 11. It has fore and aft hull andsuperstructures 13 and 15 of customary form except that the aftsuperstructure includes a refrigeration apparatus 17 which will bedescribed in greater detail presently. Ship 11 is characterized by amain deck 19, bottom plating 21, and port and starboard shell plating 23and 25.

Transverse bulkheads 27 and 29 define the fore and aft limits of themiddle body of the ship wherein the various tank spaces for the storageof cargo liquids are located. Intermediate the bulkheads 27 and 29 are anumber of other transverse bulkheads such as 31 and 33 running the fullwidth of the ship. Extending fore and aft Within ship 11 at leastbetween bulkheads 27 and 29 are'port and starboard longitudinalbulkheads 35 and 37, and extending transversely between the longitudinalbulkheads and the shell plating there may be local, noncontinuousbulkheads such as 39 and 41. The structural items of deck, bulkheads,shell plating, and bottom plating so far designated will serve to definea series of port and starboard wing tank spaces such as 43 and 45. Thesewing tanks may be used for storage of materials which are ordinarilyliquids at atmospheric conditions of temperature and pressure. Suchliquids would include various crude petroleums and petroleumdistillates.

The central part of the vessel is divided into a plurality ofcompartments such as 47 which are bounded fore and aft by continuoustransverse bulkheads such as 31 and 33, and laterally by longitudinalbulkheads 35 and 37. Within each of these compartments are located twotank structures for the storage of cold liquid cargo materials. Thesetank structures will be more completely described presently, but mayhere be said to each be characterized by an outer steel shell 49, aninner non-metallic shell 50 of a material such as polyester film innon-contacting relation to the outer shell, and thermal insulationmaterial substantially filling the region between the two shells. A coldliquid storage volume 51 is defined within the inner shell 51'9.

All of the storage regions 51 are connected to a manifold systemcomprising a main liquid filling and discharge line 53, a recondensedliquid return line 55, a vapor suction line 57 going to the inletconnection of refrigeration apparatus 17, and a vapor vent line 59terminating in exhaust pipe 61 curving downwardly over the stern, andwhich is fitted at its lower end with an exhaust head 63 wherefrom vapormay be finally vented to the atmosphere. This arrangement assumes thatthe cold liquid cargo will gasify to vapors heavier than air, as in thecase of propane.

Considering the individual connections associated with a particular tankspace 51, an admission valve 65 in a branch oil of line 53 must beopened to allow cold liquid to be filled into the tank from one of themain shore filling connections to be identified presently. An admissionvalve 67 in a branch off of line 55 must be opened to allow cold liquidto be filled into the tank from refrigeration apparatus 17 or from oneof the auxiliary shore filling connections to be identified presently.An outlet valve 69 in a branch off of line 57 must be opened to allowvapor to flow from this tank to the inlet connection of refrigerationapparatus 17 in which this vapor may be recondensed. Connection fromtank space 51 to vapor vent line 59 is made through a branch whichcontains a pressure relief valve 71. This valve is set to open atrelatively low pressure on the order of a half to one and a half poundsper square inch gauge. Thus, cargo material vapors generated within tankspace 51 which are not drawn oil through suction line 57 cannotaccumulate to any significant pressure before they escape through theatmospheric vent system, that is, through vent line 59 and exhaust pipe61, and finally out of exhaust head 63.

On the main deck 19 of ship 11 are means whereby connection may be madeto a shore facility (or another ship) to allow cold liquid to be filledinto or discharged from tank space 51. This means includes the mainshore connection valves 73 and 75 located port and starboardrespectively. These valves terminate a common line 77 runningtransversely across deck 19, and from a T-connection in this line aliquid line 79 runs directly aft to the vicinity of refrigerationapparatus 17. Connection is made from line 79 as shown through valve 81to liquid line 53. This valve and valve or valves 65 will be open whencold liquid cargo material is being filled into tank spaces 51 from ashore facility through valve 73 or 75.

Within each tank space there is a deep well pump 83 which is driven byconventional means such as a steam turbine 85 located at about the maindeck level. The discharge line of this pump is connected to liquid line53 through a valve 87. Near its aft end, liquid line 53 is connectedthrough two valves 89 and 91 to the inlet sides of booster pumps 93 and'95. These pumps are provided respectively with discharge valves 97 and99 through which connection is made as shown to liquid line 79. Whencargo liquid is to be discharged from tank space 51, one or both ofvalves 65 and 81 will be closed; valve 87 will be open; at least one setof booster pump valves 89 and 97 or 91 and 99 will be open, and one ofthe main deck valves 73 or 75 will be open also. Pressure relief valve'71 might be reset to a greater opening value in order to allow a highervapor pressure to be built up above the surface of liquid in tank space51 to insure that this liquid will be driven positively into the suctionof pump 83. Such resetting of the relief valve may be particularlydesirable when the level of liquid in tank space 51 is rather low.

Close by valves 73 and 75 are two valves 191 and 193 located port andstarboard on main deck 19. These valves terminate a common line 195running transversely across the deck. Recondensed liquid return line 55passes through a cross fitting in line 105. It may be seen, therefore,that valves 101 and 193 can be used as auxiliary connections for fillingtank spaces 51 from the shore.

Located in the forward region 13 of ship 11 is a dry inert gas source197, for example a source of dry nitrogen gas. This source may be eitheran actual gas generating plant or a bank of cylinders suitablymanifolded, and which are replenished from time to time. Running aftfrom gas source 107 is in inert gas main 109. A branch from this isconnected to each one of the compartments 47 through a pressure reducingand regulating valve 111. The particular purpose of the inert gas systemis to condition the atmosphere in compartment 47. Considering thispurpose, if there should be leakage of cold liquid cargo through bothtank shells 49 and 50 the leaked material will be vaporized by the timeit reaches space 47 outside of shell 49. If this space has an atmosphereof ordinary air, a combustible mixture of air and the cargo material maybe created, for example a combustible mixture of air and propane. On theother hand, if compartment 47 has an atmosphere of nitrogen, gaseouspropane leaking through tank shell 49 will mix with a material whichwill not support combustion.

Suitable ventilation equipment of a kind well known in the art may beprovided for compartment 47 to flush the inert atmosphere, and provide abreathable atmosphere whenever access to this compartment isdesired forparties to inspect the outer shells 49 of cold liquid storage tanks. Asuitable sniffing connection may also be provided for compartment 47 toallow sampling of the atmosphere therein. Such a connection wouldconveniently be located in deck 19.

A further possible use of the inert gas system, although not onespecifically illustrated in the drawings, would be that of applyingpressure to the surface of the cold cargo liquid in tank space 51 todrive this liquid positively into the suction of deep well pump 83, andof simultaneously maintaining a non-combustible atmosphere within thevapor space of the tank. To achieve this a connection would be made, forexample, from inert gas line 109 through a shut-off valve and a reducingand regulating valve to the tank vent branch line upstream of reliefvalve '71. After the latter had been reset to a higher than normalopening value, nitrogen or other inert gas would be admitted to tankspace 51 to aid the pumping operation. An atmosphere of substantiallyinert nature could then be maintained in the tank until a fresh cargo ofcold liquid was brought aboard. Heaviest boiling off of liquid may beexpected during the initial phases of the loading operation, and duringthat time, therefore, with tank space 51 initially filled with inert gasthe vapors surging from vent head 63 on exhaust pipe 61 would be anon-combustible mixture rather than essentially pure hydrocarbon.

Insulation of the cargo liquid storage tanks has been mentionedgenerally in connection with FIGS. 1 and 2, and will be discussed ingreater detail presently. It is obvious, however, that all of the pipingsystems so far mentioned with the exception of the inert gas lines willalso be filled with cold materials from time to time. All of the liquidlines such as 53, 55, and 79 should be thermally insulated as shown inFIG. 3 to reduce evaporative loss of cargo materials. Vapor line 57should be insulated as shown in FIG. 3 to prevent unnecessary warming ofvapors which are to be recondensed. Vapor vent line 59 may, on the otherhand, better be left uninsulated to allow warming and reduction indensity of vapors which are being dispersed to the atmosphere.

Referring next to FIG. 3, the hat plate keel of tank ship 11 isdesignated 113. Extending upwardly from this is the centerline verticalkeel 115. The vertical keel is surmounted by a platform. structure 117which is otherwise suitably braced, and which provides immediate supportfor the cold liquid tank structure of which steel shell 4? defines theouter boundary. The tank structure may be located and secured onplatform 117 by any appropriate and customary means, proper allowancebeing made for dimensional changes due to thermal effects. Pipingelements 53, 55, 57, S9, 79, and 109 already described in connectionwith FIGS. 1 and 2 are illustrated above deck 19. Particularly shown isthe valve and piping connection from inert gas line 1&9 to compartment47. Also illustrated are a connection from compartment 47 to vapor ventline 59 containing a valve 119, and a valved vent line 121 leading tocompartment 4'7 from the atmosphere.

It is by means of line 121 and the connection from compartment '47 tovapor vent line 59 that the atmosphere of this compartment may bechanged. There may be a supply blower 122 connected in line 121 toprovide the necessary air flow. Valve 119 and the valve in line 121 maybe of the spring-loaded variety to protect compartment 47 against beingoverpressured by inert gas or air on the one hand, or against beingunduly evacuated on the other.

Now considering'particularly the structure of the cold cargo liquidstorage tank and its internal attachments, the outer shell 49 will be ofsteel. Being internally insulated, this steel will preferably be of alow carbon, relatively inexpensive grade. It may of course be of astainless or other high alloy grade, but the particularly beneficialproperties of these considerably more expensive steels will not havesignificant opportunity of development in the course of such use. 0n itsinterior bottom surface, tank shell 49 is fitted with a series ofstructural elements such as inverted T-beams 123 which support a steelplate 125, and this in turn supports the bottom layer of the thermalinsulating material 127 with which tank shell 49 is lined.

The heat path from the bottom plating 21 of ship 11 to any cold liquidcargo in tank space 51 may be examined. Heat will flow in through whatmay be a fairly easy path of structural plates and shapes as far as thebottom of tank shell 49. From thereon to plate 125, however, there willbe only a narrow path available through the webs of T-beams 123. It maybe thermodynamically desirable and economically worthwhile to makeT-beams 123 and plate 125 out of some material such as stainless steelwhich in comparison with ordinary carbon steels such as that usable fortank shell 49 has a rather low coefficient of thermal conductivity. Thusit may be seen that even before any infiowing heat reaches the lowerlayer of insulating material 127, it must travel a rather difficultpath. Therefore, the rate of heat leakage into cold liquid in tank space51 is kept quite low according to the structural arrangements shown inFIG. 3.

Primary location of outer tank shell 49 with respect to the hullstructure of ship 11 is effected 'by its seating and securing onplatform 117. To prevent undue sway of this shell, however, with rollingand pitching of ship 11, it is held transversely centered by such meansas buffer brackets 129 and 131 secured to longitudinal bulkheads 35 and37 near the top of the tank shell. These brackets have no effect ofrestricting movement of the tank due to temperature changes. It is to beclearly understood, of course, that tank shell 4-9, bulkheads 35 and 37,bottom plating 2.1, shell plating 23 and 25, and deck plating 19 may andwill all be stiifened locally as needed in conformity with standardstructural and naval architectural practice.

Deep well pump 83 previously identified in connection with FIGS. 1 and 2is shown in position in PEG. 3 close to the bottom of cold liquid cargotank space 51. This pump may be of any conventional design suitable forhandling hydrocarbon liquids at low temperatures. The prime mover 85whereby pump 83 is driven is located on and above main deck 19. Thisprime mover will preferably be a steam turbine of any suitable designand including any appropriate speed reducing gearing. The use of a steamturbine is preferable to that of an electric motor in order to keep anypossible spat-king apparatus away from a deck region in closeassociation with piping carrying flammable liquids and vapors. Sleeve133 extending from turbine 85 downwardly to pump 83 has within it theturbine power transmitting means such as shafting of conventionalnature. It may also contain the discharge line of pump 83 wherethroughcold liquid cargo to be unloaded is sent to liquid line 53. This sleeveis attached by a bellows 135 to the outer tank shell 49 for maintenanceof vapor sealing of this outer shell.

The structure of the insulation layer 127 and the inner tank shell 50will now be considered. Insulation layer 127 is of at least a semi-rigidnature. Most conveniently it will comprise a plurality of well fittedblocks or slabs of insulating material secured to outer tank shell 4-9,being preferably bonded in essentially liquid-tight and gas-tightfashion to this outer tank shell and to each other. The blocks whichinsulation layer 127 comprises may be the traditional balsa or cork orother materials of the prior art such as insulating concrete. They mayalso, however, and indeed preferably will be blocks of prefoamed, closedcell plastic.

Because of their cellular structure and large number of dead air spaces,plastic foams are ideally suited as thermal insulations. In addition toproviding low thermal conductivity, these materials are also recommendedbecause of their high strength-weight ratio, their low odor, their goodresistance to water vapor, and their case of fabrication. in general,the thermal properties of plastic foams are determined by resin content,density, and type and size of cellular structure. Among the plasticmaterials which may be prefoamed to rigid or semi-rigid blocks arepolystyrene, urethane, and polyvinyl chloride. All of these materialsmay be used as insulation at temperitures as low as 50 F. without dangerof their becoming unduly embrittled and susceptible to crumbling understress or impact. Another characteristic of these plastic materialsprefoamed in block form, even those designated as rigid plastics, isthat of at least a residual amount of flexibility. With this flexibilitythey can accommodate themselves to at least slight changes in shape andsize a of outer tank shell 49 due to normal working of vessel 11.Insulation layer 127 will be about two inches thick for cold liquidcargo temperatures of about '0 F.

To effect the above-mentioned liquid-tight and gastight bond between theblocks of insulation layer 127 and the outer tank shell 49 and betweenthe blocks themselves, it is proposed according to this invention to usea novel Thiokol-epoxy compound having generally good hydrocarbonresistance, adhesive properties, thermal insulating qualities, andflexibility at low temperatures, that is, at about -50 F. TheThiokol-epoxy bonding or sealing material of this invention may becompounded using the following commercial ingredients in the exemplarypercentages indicated:

Weight percent LP-3 Thiokol (Thiokol Corp.) 200 Epon 828 (Shell ChemicalCo.) 100 DMD-30 Amine (Rohm & Haas Co.)

Thiokol LP-3 is one or a series of liquid polymers of varying viscositycapable of being converted to tough resilient rubbers at roomtemperature Without appreciable shrinkage. Other polymers of the seriesinclude those designated LP-Z, LP-8, LP32, and LP-33. The cured(converted) liquid polymers are resistant to oil, most commercialsolvents, and water swelling. They have good electrical resistivity andaging characteristics. They retain a considerable degree of flexibilityat temperatures as low as 65 F. as well as the ability to Howplastically under stress. Other desirable characteristics of thesepolymers include high impermeability t0 gases and moisture, resistanceto ozone and sunlight, and strong adhesivity to may materials.Particular properties of LP-S Thiokol are as follows: viscosity at 77 F.(approx.), 10 poises; molecular weight (approx.), 1000; pour point, F;flash point (open cup), 418 F.; fire point (open cup), 465 F, andmoisture content (max), 0.1%.

Epon 828 is one of a series of synthetic resins possessing terminalepoxide groups. These resins range from mobile liquids to viscousliquids to solids, Epon 828 itself being a viscous liquid. The primarydifierence among the various types is the molecular weight whichincreases as the identifying ,number increases. Polymerization andcopolymerization reactions of the epons can be effected through eitheror both of their reactive groups, that is, the epoxide groups and thehydroxyl groups. By the addition of small amounts of organic polyamines,epon resins can be polymerized to form clear, light-colored, toughproducts with high physical strength and chemical resistance.

DMP- amine is a dimethylaminomethyl phenol which is used as a catalystin the compounding operation whereby the Thiokol-epoxy bonding andsealing material of this invention is formulated. The particularexemplary percentages indicated may, of course, be varied as desired togive variations in physical properties of the compounded material, solong as final set is obtained in a reasonable time such as abouttwenty-four hours.

To the mixture of LP3 Thiokol, Epon 828, and DMP- 30 amine is added upto an equal volume of lightweight, hydrocarbon resistant material offine gradation to permit troweling. A finely divided material suitablefor this purpose is one comprising a large number of very tiny particlescommonly called microballoons which are best known as a means forproviding a non-rigid continuous coating of some thickness on thesurfaces of hydrocarbon liquids in storage vessels to reduce evaporationlosses of these liquids. The particles are of low density and float onthe liquid surface.

' A species of such particles or microballoons are nitrogen-filledmicrospheres of thermo-setting plastic. They may be made fromphenol-formaldehyde resin or other oil-insoluble resins, polyethylene,etc., according to the process disclosed in US. Patent No. 2,797,201issued to F. Veatch and R. W. Burhans on June 25, 1957. Thesemicrospheres may have an average diameter of 1' to about 500 microns,preferably 25 to 250 microns. Their bulk density is within the range of0.01 to 0.3, preferably 0.1 to 0.2, and liquid displacement densitywithin the range or" 0.05 to 0.6, preferably 0.2 to 0.5.

In building up the insulation structure shown in FIG. 3, the individualinsulating blocks of layer 127 will be coated on their abutting surfaceswith the Thiokol epoxy compound described above as with a mortar, andlaid up like brickwork. The abutting surfaces of these blocks willinclude both those bearing against the inner surfaces of outer tankshell 49 and plate 125, and those giving bearing block-to-block. Theremay, of course, be some primary locating means for the blocks, such asstuds 12% set on tank shell 49 and plate to enter prepared holes in theblocks. If such studs or other locating means be used, however, theymust be of such material (stainless steel, for example) and such shortlength to create no easy path for the flow of heat into tank space 51.

With the blocks of insulating, layer 127 in place and bonded to outertank shell 49 and to each other, the inner tank shell 5% may be erected.A highly desirable material for this inner shell is a polyester filmknown commercially as Mylar, and manufactured by E. I. du Pont deNemours Co. This film comprises essentially a highly durable,transparent, Water-repellent polyethylene terephthalate resin. it ischaracterized by outstanding strength and chemical inertness. it ischaracterized further by a high degree of flexibility even attemperatures far below -50 F., an attribute of great importance forpurposes of the present invention. Mylar is available in thicknesses upto about ten mils (6.01), and in strip widths up to about fifty-fourinches. Strips of this material may be joined to each other by buttfusing or Welding upon application of heat as from a hot iron, or bylapping and the use of a suitable adhesive material.

In erecting inner tank shell 50, it will, of course, be necessary toobtain a firm edge joint between abutting strips of Mylar to insure thecreation of a tank shell that is both gas and liquid tight, and it Willat least be highly desirable to obtain a bond between the Mylar stripsand insulation layer 127 to prevent any collapse or significant degreeof deformation of inner tank shell 50 once it has been fabricated. Sucha bond may be obtained by applying a light coating of theabove-described Thiokol-epoxy adhesive compound to the inner surfaces ofthe insulation layer 127 before the polyester film strips are set inplace on these surfaces. The film strips may be either butted or lappedalong their mating edges, and joined by heat or an adhesive materialsuch as a Thiokol-epoxy to complete the fabrication of a gas-tight andliquid-tight inner tank shell 50.

In order to insure that there will be a two-shell barrier all aroundtank space 51, inner shell 59 must be tightly but flexibly joined topump sleeve 13 3. This joint can be achieved, among other Ways, by useof a polyester film bellows 137 adhered to or formed integrally withtank shell 50, and secured to the pump sleeve by a suitably clampingring 139. It may be seen, therefore, that by the use of bellows elementsand 137 the cold liquid storage tank structure of this invention mayilex quite freely due to temperature variations, working of vessel 11,or any other reason without there being danger of rupture of thedouble-shell barrier surrounding the cold liquid cargo or of any unduestressing of structural parts.

Inner tank shell 5% will preferably and most conveniently be ofpolyester film, that is, of Mylar. It may, however, be formed of thesame Thiokol-epoxy compound used as a mortar and sealing agent betweenindividual blocks of insulation layer 127, and between these blocks andthe inner surfaces of outer tank shell 49 and plate 125. This compound,as before noted, is made up with an aggregate of microballoons to giveit consistency for troweling. It may thus be spread on the innersurfaces a? of insulation layer 127 to a finite thickness, a thicknessof one quarter inch for example, and allowed to set to form a tank shellstructure in and of itself. For completion of this shell structure, aseparate bellows 137 and clamping ring 139 could be used as shown inFIG. 3 to make the final joint to pump sleeve 133.

Referring finally to FIG. 4, What is shown is a refrigeration systemgenerally identified as 17 in FIGS. 1 and 2 which uses, for example,propane as a working substance. Only a part of the propane operates in aclosed cycle. The net material infiow of the whole system is thegassedoif propane collected from cold liquid cargo tank spaces 51through vapor line 57, and the net material outflow is substantiallythis same amount of propane recondensed to a liquid flowing back to tankspaces 51 through liquid line 55. The operation of this refrigerationsystem will be described in terms of a numerical example.

Assume that a pressure of 16.2 p.s.i.a. (1.5 pounds of positivepressure) is maintained in tank spaces 51. The liquid propane stored inthese spaces and the vapors arising therefrom will then be at atemperature of -40 F. These vapors are taken through line 57 toknock-out drum 14-1 which is fitted internally with steam coil 14-3.This drum and steam coil have the purpose of vaporizing any droplets ofliquid propane which may be carried over from tank spaces 51. From theknock-out drum the now fully vaporous stream of gassed-cit propane flowsto the first stage inlet of a compressor unit comprising first stage145, second stage 147, and prime mover 149. In this first stage thepropane is compressed to about 59.5 p.s.i.a. At the outlet of firststage 145, the vapor which has come back from cold liquid cargo tankspaces 51 is mixed with additional propane vapor about half its ownweight or rate of flow. This additional propane vapor, representing thatquantity of propane operating in a closed cycle, arises from flashchamber 151.

From the mixing point of the vapor streams from the first compressorstage 145 and flash chamber 151, the combined weight of propane flows toand through the second compressor stage 147, being therein increased inpressure to about 226 p.s.i.az. Leaving the second compressor stage, thehigh pressure propane flows to and is condensed in the water-cooled heatexchanger 153, and from there is collected in receiver or surge tank 155as a liquid at about 226 p.s.i.a. and 115 F.

A line connecting the bottom of receiver 155 with about the mid-heightof flash chamber 151 and provided with a suitable stop valve 157 andthrottle valve 159 allows a controlled flow of liquid propane from thereceiver to the flash chamber. As it flows through this line and thesevalves, particularly through valve 159, the liquid propane is reduced inpressure down to about 59.5 p.s.i.a., and about one third of it flashesaway to vapor. This vapor fraction rises through the stop-check valve161, and mixes with the outlet stream from the first compressor stage145 to form the inlet stream to the second compressor stage 147.

In steady operation of the system, a certain level of liquid propane atintermediate pressure (59.5 p.s.i.a.) will be carried in flash tank 151,just as there will be a certain level of liquid propane at high pressure(226 p.s.i.a.) carried in receiver 155. A liquid discharge line, whichbecomes the liquid return line 55 on the downstream side of throttlevalve 163, leads away from the bottom of flash chamber 151. Saturatedliquid propane flowing through valve 163 on the way back to tank spaces51 is reduced in pressure from about 59.5 p.s.i.a. to about 16.2p.s.i.a.

Some of this liquid, approximately 20%, flashes to vapor, so that thematerial arriving back at tank spaces 51 through line 55 is not allliquid but is rather a mixture of liquid and vapor. The gaseous fractionof this stream is, of course, returned to the refrigeration systemthrough vapor line 57 along with that vapor which represents propaneactually gassed-ofl due to heat leakage into tank spaces 51. In itsnormal utilization, refrigeration apparatus 17 will be most heavilyloaded when tank spaces 51 are being filled with cold liquid cargo. Theprior mention of possible use of the inert gas system for blanketing andpurging these spaces during filling operations does in no way excludeemployment of the recondensation system just described for accommodatingvapors boiled oil from the cold cargo liquid at this time.

Although this invention has been described with a certain degree ofparticularity, it is to be understood that the present disclosure hasbeen made only by way of example, especially with regard to numericalquantities given herein, and that numerous changes in the details ofconstruction and the combination and arrangement of parts may beresorted to without departing from the spirit and scope of thisinvention as hereinafter claimed. In panticular, it is to be understoodthat when inner tank shell 50 is made of polyester film it may be givenany desired wall thickness by cementing together in superimposedrelation a plurality of film sheets. Such cementing may be effected withthe Thiokol-epoxy compound described above. prefabrication of thepolyester film inner tank shell 5% may be carried out depending upon theconstruction sequence chosen for erection of the cold cargo liquidstorage tank. Such prefabrication would be in contrast to laying upindividual inner tank shell panels of polyester (Mylar) film after thecold cargo liquid tank structure had otherwise been substantiallycompleted.

What is claimed is:

l. A thermally insulated tank structure for containing low temperatureliquids, said tank structure being adapted for installation in marinevessels and comprising a first shell member of a low carbon steel whichis susceptible to reduction in its impact resistance property at thenormal low temperature of said liquid, at second shell member of aT'niokol-epoxy compound admixed with an ag- 2. A thermally insulatedtank structure :for containing low temperature liquids, said tankstructure being adapted for installation in marine vessels andcomprising a first shell member of a metal which is susceptible toreduction in its impact resistance property at the normal lowtemperature of said liquid, a second shell member of a plastic,nonmctallic material which retains a significant degree of-flexibilityat the normal low temperature of said liquid, said second shell memberbeing located within said first shell member and in spaced relationthereto, thermal insulation material substantially filling the spacebetween said first and second shell members, said thermal insulationmaterial comprising a plurality of at least semi-rigid prefoamed plasticblock elements in abutting relation one to another, and an adhesivematerial of a Thiokol-epoxy compound admixed with an aggregate substancebetween said blocks and said shell members whereby an essentiallyliquid-tight and gas-tight seal is made between said blocks and saidfirst and second shell members.

3. A thermally insulated tank structure for containing low temperatureliquids, said tank structure being adapted for installation in marinevessels and comprising a first shell member of a metal which issusceptible to reduction in its impact resistance property at the normallow temperature of said liquid, a second shell member of a plastic,non-metallic material which retains a signifi- Likewise, any convenientdegree of cant degree of flexibility at the normal low temperature ofsaid liquid, said second shell member being located within said firstshell member and in spaced relation thereto, thermal insulation materialsubstantially filling the space between said first and second shellmembers, said thermal insulation comprising a plurality of at leastsemi-rigid block elements in abutting relation one to another, and anadhesive material between said blocks and said shell member-s whereby anessentially liquidtight and gas-tight seal is made between said blocksand said first and second shell members, said adhesive materialcomprising an admixture of about equal volumes of an aggregate substanceand a Thiokol-epoxy compound including about one hundred par-ts byWeight of a synthetic resin possessing a terminal epoxide group andabout two hundred parts by weight of a liquid polymer.

4. A thermally insulated tank structure according to claim 3 in whichsaid aggregate substance comprises essentially microspheres ofthermosetting plastic.

5. A thermally insulated tank structure for containing low temperatureliquids, said tank structure being adapted for installation in marinevessels and comprising a first shell member of a metal which issusceptible to reduction in its impact resistance property at the normallow temperature of said liquid, a second shell member of a plastic,nonmetallic material which retains a significant degree of flexibilityat the normal low temperature of said liquid, said second shell memberbeing located within said first shell member and in spaced relationthereto, thermal insulation material substantially filling the spacebetween said first and second shell members, said thermal insulationmaterial comprising a plurality of at least semi-rigid block elements inabutting relation one to another, and an adhesive material between saidindividual blocks and between said blocks and said shell members wherebyan essentially liquid-tight and gastight seal is made between individualabutting blocks and between said blocks and said first and second shellmembers, said adhesive material comprising an admixture of about equalvolumes of an aggregate substance and a. Thiokol-epoxy compoundincluding about one hundred parts by weight of a synthetic resinpossessing a terminal epoxide group and about two hundred parts byweight of a liquid polymer.

6. A marine vessel for bulk transportation at substantially atmosphericpressure of liquefied materials which are normally gaseous atatmospheric pressures and temperatures, said vessel comprising a basicstructural hull and at least one tank structure mounted in said hull andoccupying a substantial portion of the interior volume thereof, saidtank structure including a first shell member of low carbon steel, 21second shell member of a plastic, non-metallic material which retains asignificant degree of flexibility at temperatures down to at least about-'-'G F. disposed in spaced relation to said first shell member withinsaid first shell member, thermal insulation material intermediate saidfirst and second shell members, said insulation material comprisingessentially a prefoarned plastic, and a Thiokol-epoxy adhesive materialbetween said insulation material and said shell members whereby anessentially liquid-tight and gastight seal is made between saidinsulation material and said first and second shell members.

7. A marine vessel according to claim 6 in which said second shellmember comprises essentially a polyester film.

8. A marine vessel according to claim 6 in which said second shellmember comprises essentially a Thiokolepoxy compound admixed with anaggregate substance.

9. A marine vessel according to claim 6 which includes conduit means forsupplying said liquefied material into said second shell member of saidtank structure, pump means for discharging said liquefied material fromthe lower portion of the space within said second shell member, andconduit means connected with the upper portion of the space within saidsecond shell member for atmos pheric venting of vapors arising from saidliquefied material within said second shell of said tank structure.

10. A marine vessel according to claim 9 which includes conduit andrefrigeration means whereby said vapors arising from said liquefiedmaterial within said second shell of said tank structure may begathered, reiiquefied, and returned in a substantially liquid state tosaid space within said second shell member preferentially to ventingsaid vapors to the atmosphere.

11. A marine vessel according to claim 9 which includes a source of dryinert gas and conduit means for injecting gas from said source at apressure at least slightly greater than atmospheric into said hullstructure eXteriorly of said first shell member of said tank structureto provide an atmosphere of a substantially inert nature in contact withsaid first shell member.

